Method for continuously transforming a dispersion of a rubbery material into sheets



' W. BIXBY F, 2,424,648 METHOD FOR coNTINUoUsLY TRANSFORMING ADIsP'ERsIoN July 29, 1947.

0F A RUBBERY MATERIAL INTO SHEETS Filed May 9, 1942 lull ww @@wvw wwwmm1 mmm.

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,WTHD Fi FOR@ d DEWERSEN 0F A RUBBEEY MEMBER@ ENT@ SET@ triunfe r. naar,ou, one, naar to 'rae E. IF. Goh diompany, New York, N. Y., a

corporation oi New ilork Application li/iay it we?, Serial No. 442,350

(ci. eco-23) n C. E This invention Y relates to a method forconftinuously transforming a dispersion of a rubbery synthetic rubberlatex is ordinarily only from one-tenth to one-third as great -as theparticle size of Hevea latex.

It is an object ofthis invention to provide apa y paratus and a methodfor the continuous coagula- The method of this invention may be appliedto anydispersions of robbery materials. Emul sion polymers ofbutadienes-l,3 such as butadiene-1,3, isoprene, piperylene,2,3-dimethylbutadiene orl 2chlcrobutadiene1,3 or copolymers ofbutadiene-1,3 with monoethylenic monomers copolymerlzable therewith inaqueous emulsion such as aorylonitrile, styrene, methyl methacrylate, ormethyl isopropenyl ketone, prepared ilo lib

tion, Washing, and drying of a dispersion of a y robbery material toform a uniform sheet. It is -a further object'to provide a methodwhereby a dispersion of a rubbery material can. be coagulated to form aslurry containing small 'rubbery It is a further object to provide amethod for preventing the agglomeration of the small robbery particlesformed. It is. a further object to provide a method for decreasing thesizeof large particles in a slurry oi' rubbery par# ticles. It is afurther object to provide a method ofwashing rubbery particles on apervious carrier without disturbing their position. it is a furtherobject to provide a method for-pressing mbbery particles into asheetupon a pervious carrier Without causing the adhesion of the particles tothe carrier. Further objects of the invention will be apparent from thefollowing detailed'description of the apparatus and a continuous methodfor transforming a specific synthetic rubber latex 'into a sheet.

Much rof the diculty which has heretofore attended attempts continuouslyto coagulate and sheet dispersion of rubbery'r materials has beenexperienced in attempts to wash the coagulum free from the emulsifyingagent and other ma terials present during the polymerization which arenot desired in the product.' `Ihav'e now discovered' that the dicultiesare largely avoided if a dispersion of a rubbery material is coagulatedto form an' aqueous slurry 'of uniformsmall, A rubberyv crumbs, theslurry is deposited upon a4 sheet, dewatered, and dried. f f

in alkaline emulsions with a soap such as sodium myristate, sodiumpalmitate, sodium oleate, or a 'mixture of soaps are preferred materialssince they can be readily coagulated to form a slurry of uniform smallcrumbs less than 1 mm. in diameter by adding a lsalt to the latexandthen lowering the pH to edect the coagulation. A Butadienel,3 copolymersprepared in the presence 'of acid-stable emulsifying agents such assodium lauryl sulfate may be coagulated f in the form of uniform, smallcrumbs by merely adding a salt f solution to the latex with stirring.'likewise,

Hevea latex or artificial dispersions of natural or reclaimed rubber maybe employed if they are compounded in such a manner that they can becoagulated to form a slurry of uniform, small crumbs. i

The invention may be better understood from the accompanying drawing ofwhich Figs. 1a, b and c illustrate apparatus suitable for coagulatlng,washing, sheeting and drying dispersions ofrubbery materials.

Referring to the drawing, the formation of the slurry of small, rubberycrumbs is performed in tanks iii, il and i2. Two storagetanksid andcentrifugal pump i5. Conduit i6 leads from the pump it to acrossvalve ilwhich distributes the contents of tank In between conduit it leadingback into the tank It and conduit it leading into the coagulating tankil. The treated latex ilowing from the conduitlt is lntermingled withfresh latex flowing from storage tank id through conduit 2t, and fallsin to tank lll which is provided 4with a stirrer 2l.; Coagulating tank il also has an outlet leading into another open impellor centrifugalpump'22 from whichva conf duit Zilleads to crossvalve 24 which dividesthe contents of the Aconduit between tanks Il and l2 through conduits 25and 26. The contents of conduit 25, are intermingled with coagulantsolution flowing from supply tank 2l through conduit 28 and fall intotank i l. which is provided with a stiri'er 29. Tank l2 is also providedwith a stirrer 3 30, and has an outlet leading -into another openimpellor centrifugal pump 3| which discharges through conduit l2 intocross valve33 dividing the contents between conduits 34 and 35. VThecontents of conduit 34 are intermingled with the pH-controlling materialof supply tank 36. discharged through conduit 31, and returned to tankI2. Conduit 35 leads into a mechanical disintegrating device comprisinga stator 38 pro- I vided with inwardly-extending knives 39 and adispersed inthe water. The stabilizing is conamount suiiicientto'increase -the I /particle size of the latex but insuilicient to causeirreversible rotor 40 provided with outwardly-extending e knives 4I. Theconduit 42 leads into the head box 43 provided with a distributing roll44 nand the weirs 45, 46. An apron 41 extends from the head box to thesheet-forming portion of the apparatus. f

The sheet-'forming portion of the apparatus consists of an endless 'wiremesh screen 50 passing around driven rolls 5I and 52 and provided with.tensioning 'rolls 53, 53 and cleaning rolls 54, 54 consisting of wirebrushes'. Spray pipes 56, 56 also assist in keeping the screen clean.Water coagulation as disclosed in the copending application of Edward A.Willson, Serial No. 442,368, died May 9, 1942. y The amount of saltemployed will depend upon the concentration and nature oi the latex, theconcentration and nature of the emuisifyig agent in the latex, the pH ofthelatex, and the particular salt added. For a synthetic rubber latexcontaining between 20 and 40% by weight of a copolymer of butadiene andstyrene and from 3 to 8% based on the rubber of soap, fromA 10 to 50% byweight of sodium chloride basedon the rubber are ordinarily required toachieve the desired'increase in stability. Latices in which the soap isless soluble, as. iorinstance,

'1 those resulting from the emulsion copolymerizafor washingthe materialon the screen is sprayed from the nozzles 51, 51 in pipe 58 onto thespreader pans 59, 59, from whence it flows upon the material on thescreen. The force of the spray is broken by the spreader pans 59, 59.Pieces or cloth 60 placed on the lower edges of the spreader pans, lorscreens (not shown) placed on the spreader pans at right angles to the'direction of and styrene in which the soap is quite soluble.

now of the water may be employed to prevent channeling and assist inobtaining an even distribution of water.l The washing means below thescreen is located further along in the direction `of motion of thescreen than the washing means above the screen and consists of thenozzles 6 I, 6| in the pipe 62. The screen passes under a dandy roll 63which isa hollow wire screen roll containing the spray 64, under therubber press rolls 65, .65 after each of which is located washing meansconsisting of nozzles 66, 66 in a pipe 61 above the screen and nozzles68, 68 in pipe 69 located under the screen. The screen then passes underthe rubber couch roll 10'whose center rotation is slightly nearer thehead box than the center of rotation of the driving roll 52.

Further pressing of the material leaving the screen is achieved bypassing the material on the felt 15 passing around the rolls 16 and 11and the tensioning rolls 88, 88. The material on the felt is dewateredby passing between roll 89 and driven roll 90, roll 89 being a woodenroll and roll 90 Y being a suction roll. and by passing between thesolid metal rolls 9| and 92. .Y

The material passed from the felt 15 to the conveyor |00 driven by rollsI0| and |02 which passes through an oven |03 provided with hot air Yducts |04, |04 above and below the conveyor. In

The salt is added in the form of an aqueous solution, preferablycontaining from 5 to 15% of salt. Although more concentrated and evensaturated salt solutions may be employed, it is more difficult' to avoidthe presence of high local concentrations of sait than whendilutesolutions are used. J

When the apparatus herein described is in operation, brine is containedin storage tank I3, latex lis 'contained in storage tank I4, and tank I0contains stabilized latex. Operations are ordil narily started bypartially filling the tank I0 with water, then starting the flow ofbrine from tank I3. The pump is then started, and when the properconcentration of brine in tank I0 has been reached, the latex flow fromtank I4 is started.'

After a time, thesystem comes to equilibrium. Theaddition of the saltincreases the size and decreases the surface of the rubbery particleswith the `result that the stability of the latex is I' increased withoutany increase in the amount -copolymerization of '15A parts by weight ofbuta- 1 diene and 25 parts of styrene in the presence of.

5 parts' o1 soap (sodium oleate). The latex contained about 30% byweight of synthetic rubber. Such a latex is rather unstable and can bepumped only with dimculty. The irst step in the operations isaccordingly a stabilizing operation whereby the pmicle sizeof thesynthetic rubber in the latex is increased while the particles stillremain lof emulsifying agent. The latex prior to the addition of thesalt has an average particle size of only about 0.1 or 0.2 micron, butthe stabilized latex` has 'an average particle size of about 0.4 micron.The brine from tank I3 is preferably introduced into the stabilizedlatex at or near the entrance' to the centrifugal pump I5, so that anintimate admixture will occur soon after addition of. the brine and highlocal concentrations of salt willbe prevented. The stabilized latex isthen recycled through conduits I6 and I8 and led back into tank I0.Supply tank I4 is filled with latex which is added in suilicient volumeIto' keep the supply of latex in tank 'I0 substantially constant.Impingement mixing is obtained by contacting the streams of untreatedand turbulentlyilowing salt-treated latex issuing from conduits 20 andI8 respectively, the velocity aand amount of the salt-treated latexbeing considerably (greater than thevelocity and amount of the untreatedlatex. Other means oi introducing a relatively small amount of untreatedlatex into the turbulently-owing salt-treated latex may be employed.Thus conduit 20 may lead directly into conduit I8, andthe treated latexflowing turbulently through the conduit may ,entrain the untreated latexand -then pass into the tank I0. If desired, the pipes 20 and I8 candischarge beneath the surface of the liquid in tank I0, or `can leadinto a common pipe which discharges beneath the surface.

Although satisfactory results are obtained by adding a simple saltsolution to stabilize the latex,

additional advantages are obtained by including a buffer such as sodiumbicarbonate, sodium acetate, sodium tetraborate or sodiumacidphosphatein the brine solution in amounts considerably smaller than thesalt. It is desirable in some cases at a later stage in the process tohave presl a protective agent such as sulfonated rosin and/or sulfonatedpine oil. The mixing may be efllciently performed by adding the brineand the latex at a rate of 2 gallons per minute through conduits 1A" ininternal diameter, recirculating the late'x through conduit I6 at a rateof 150 gallons per s acid-stable wetting agents such as long chain alkylsulfates including sodium lauryl sulfate. al-

kyl .aryl sulfonates such as sodium isopropylJ "naphthalene sulfonate,aromatic or long-chain alkyl compounds containing a, polyalkoxychainterminating ina. hydroxyl group, or sulfonated terpenes such as'sulfonated rosin or `pirie oil, or protective colloids such as animalglue. gel- 1 atin, dextrin, stai-ch, colloidal clay, etc. These minute,diverting 4 gallons per minute through conduit I9, and recirculating 146gallons per minute of stabilized latex through conduit I8 which may beabout 2 in internal diameter.

The coagulation is effected in tank Il which, at the start of theprocess is lled with coagulated latex slurry or water. The salt-treatedlatex entering the tank is mixed with the coagulated latex having a pHof about 8.5. The contents of the tank are'pumped at the rate of 150gallons per minute through conduit 23, with 5 gallons per minute beingse-nt through conduit l26 and the remainder passing through conduit 25and contacting the 'coagulant ,iiowing from supply tank 21 throughconduit 28 at a rate of 1 gallon per minute. Again, the high velocityand quantity of ow of the slurry issuing from pipe 25 as compared withthe coagulant issuing from pipe 28 is important. The streamsintermingle, and then fall into the coagulating tank Il. In this casethe coagulant is 0.25% sulfuric acid, although acetic acid or othercoagulants in suitable conprotective agents are added to the latexbefore coagulation or to the slurry after coagulation in an amountsufdcient to prevent the agglomerav tion of the rubbery crumbs.

The details of the next step in the process vary depending upon vwhetherfatty acid 1s desired in the product or not. `If it is -desired toprepare synthetic rubber containing a substantial amount ofV fatty acid,the pH of the slurry of -rubbery crumbs is reduced to about 4 to convertthel soap into fatty acid. This may be done in tank i2, the slurrybeing` recirculated at a rate of 150 gallons per minute, 5.6 gallons ofslurry being diverted through pipe 35 and the remainder passing throughpipe 34 and intery mingling with ,dilute acid being fed from tank 36through pipe 31 at a rate of 0.6 gallon per minutemA concentration ofsulfuric acid of l or 2% is ordinarily suilcient to maintain the pH ofthe slurry at 4 or below.A During this process, the rubbery crumbs growto a size of from 0.5to0.8mm..

If the presence of fatty acid is not desired in the`flna1 product,l theaqueous slurry may b washed in tank I2 to extract as much of the soap aspossible from the rubber particles. This washing operation is preferablyperformed 'at a centrations may also be employed. The same modificationsin the mixing of the coagulant with a relatively large volume ofturbulently-flowing slurry may be employed as in the mixingof theuntreated latex with the salt-treated latex.

It is essential for the successful operation of the continuous processhereindescribed that the synthetic rubber latex be coagulated in theform of uniform small particles, preferably under 1 mm. in diameter. Inthe particular method of coagulation herein described," aturbulentlyflowing stream of the mixture of coagulated latex slurry anda small amountof uncoagulated salt-treated latex is mixed with a Asmallstream of coagulant in such a manner that a high local concentration of.either coagulant or uncoagulated latex is avoided. The particles thusformed range between 0.2; and 0.5 mm. in diameter. After the syntheticrubber has been \coagulate'd in the form of uniform small particles, itis de sirable to have present a protective agent to keep them fromclumping together to form larger particles. If the lsubsequent treatmentissuch thatl the. emulsifying agentv is immobilized ,ory destroyed,additional protective agent shouldbe added.v The protective agent may beanyv of thel cduit into a container.

convert continuously a dispersion of a rubbery.

pH of from 8 to 10. A solution of sodium hy.- y

droxide is added from tank 35 of a proper concentration to maintain thepH at the desired level. Additional tanks may be employed to Wash thecrumbs if desired.

If desired, the concentration of acid being fed from tank 21 can be highenough both t0 coagulate the latex and to convert the desired amount ofsoap into fatty acid. It has been found, however, that a somewhat moreuniform slurry may be obtained by carrying on the coagulation andconversion of the soap as separate steps in the process.

The stabilization and coagulation of the dispersions o f rubberymaterial in separate ysteps as shown above assists in the formation of aslurry of uniform, flnely-divided rubbery crumbs, :but it is notessential that'these steps be carriedpn separately. Thus the syntheticrubber lateximay be caused to entrain streams of salt solution; andcoagulant solution either successively .prf.,simul taneously while4flowing turbulentlyathrough a conduit. Also, a. turbulently-iiowingslurrymay be caused to entrain streamsv of uncoagulatedber latexencounters jets of salt solution and coagulant solution as itis fallingfrom a con- Other means adapted to material into a slurry of smallrubbery crumbsJ may also be employed.

If the coagulation of the latex has bee carried on improperly and thesizeof the crumbs is not uniform or if there are agglomerates over 14mm. in diameter inthe slurry, it is desirable prior tothe washing stepto reduce mechanically the size of the crumbs. without reforminganun- Ymaterial.

ical disintegrators employing sets of knives or corrugated surfacesbetween which the crumbs are disintegrated may be employed. It is highlydesirable that a protective agent be included in the slurry before it isfed through a mechanical disintegrator. If the coagulation has beeneffected in such a manner that the slurry consists of uniform particlesnot over about l mm. in diameter, however, thereds 'no necessity foremploying a mechanical disintegrator.

The slurry is next led into head box 43 provided' with a distributingrou 44 which aids in maintaining an even distribution of the crumbs inthe slurry and assists inpreventing further increase in particle size.,The distributing roll herein shown consists of a cylindrical sheetmetal member containing holes `on 1/2" centers, although any other kindof perforated roll may be employed.

From the head box, the slurry passes over the weirs 45 and 46 and ontothe screen 50. This screen may be made of any desired pervious A 70 meshscreen made of Duraloy wire (a nickel-copper alloy)` has been found tobe very satisfactory for the purpose since it is acid-resistant, and anysynthetic rubber adhering to the screen can be burned therefrom takingproper precautions not to harm the wire. The crumbs form a porouslayerupon the screen, and the washing process is then started. Thetimpact of water'being sprayed directly on the crumbs at this stage isapt to disturb the layer, so the Water is preferably sprayed on a curvedpiece of metal that the temperature be below at least 100 F.

Water of a temperature of about 60 F. was em' sployed in the operationsherein described. After passing under the roll 65, a jet ofcold water,air, or other iid from nozzle 68 underneath the screen lifts the sheet.momentarily from the screen to prevent the sheet from permanentlysticking thereto. The sheet isthen passed under another rubber pressroll 65, is 'washed with' cold Water from nozzle 65 directed at the biteof the roll, and is again lifted from the roll by another spray of coldwater, air or other fluid before'passing under the next roll. Thesepressing, washing,

50 from which a stream of water flows upon the' 1 moving layer of crumbswhile progressing at about the same velocity and in approximately thesame direction. The force ofthe oW may be further broken ,by attaching apiece of woven or felted fibrous material 60 to the lower end of thetrough 59. At this stage of the process, it is advantageous to employhot wash water. The water may be at any desired temperature as from12o-200 F. In the particular operation herein described the washwaterwas heated to about 140 F.

After several Washes from the top of the screen, the synthetic rubberparticles have cohered to such an extent that they can be washed frombeneath the screen Without disturbing their position. This is effectedby the sprays of hot water from the nozzles 6|, 6| shown in the drawing.The sheet then passes under the dandy roll 63, which is a hollow metalroll having a perforated face allowing the free passage of Water intothe interior of the roll and preventing the formation of a bank of waterin the bite of the roll which might break the sheet. The sheet thenpasses under the rubber press roll 65, and is then washed by a'jet ofcold water from nozzle 56 directed onto the sheet at the point where thepressure of the roll is removed. The sheet is in a distorted conditionat this point, and absorbs Water more readily than when the strainsimparted to it by the roll have disappeared. The .use of cold Water at,this point is a very important factor in the Aformation of a sheet ofsuflicient strength to permit' theremoval of the sheet from the screen.Although reduction of the temperature to 32 F.

is not necessary, the water should be as cold as economically feasible,and it is very desirable and lifting operations may be repeated anydesired number of times. The sheet then passes under a top couch roll l0centered slightly ahead of the driving roll 52.

The continuous sheet now contains about of water onv a dry rubber basis,and is removed from the screen and to a carrier 15 which is preferablyconstructed of pervious, hapless' -brousx material such as a naplessopen-woven wool fab ric. The sheet passes between press roll 89 andsuction roll 90 where 50% of the water is removed, and then throughpress rolls 9| and 92 where 25% more of the original water is removed.I'hese values apply to the specific synthetic rubbei` employed in thisexperiment, but they may vary widely with the type of synthetic rubberand the amount of fatty acid left in the synthetic rubber. otherconveyor |00, which should support and carry the sheet withoutcontacting it over any greater area than necessary, and is carriedthrough a hot air drying oven |03. The hot air blows upon the top andbottom of the sheet through ducts |04, |04, and may be of any suitabletemperature from to 320 F. or higher, for instance, depending upon theparticular synthetic rubber being dried. The sheet then passes intoanother chamber where cold air is blown upon the sheet through ducts|05, |05, passes out of the oven.' and onto a wind-up roll. Alternately,the sheet may be festoonedand pressed into bales. The product is in theform of a uni-y form, coherent sheet having `a, pleasing appearance andis very easily broken down upon a roll mill.

I claim: l

1. The method of continuously transforming an alkaline soap-containingaqueous dispersion of a synthetic rubber comprising a polymer of abutadiene-1,3 in the form of particles under 0.3 micron in diameter,into a synthetic rubber sheet, which comprises adding to the dispesion awatersoluble salt of a water-soluble acid in an amount suflicientto-increase the size of the particles in and the stability of thedispersion but insufficient to coagulate the dispersion, admixing thesotreated dispersion with an aqueous acid coagulant to form an aqueousslurry of`discr'ete synthetic rubber crumbs and continuously forming thecrumbs into a sheet.`

2. The method of continuously transforming an alkaline soap-containinga'queous dispersion of a synthetic rubber comprising a polymer of abutadiene-1,3 inthe form of particles under 0.3 micron in diameter, intoa synthetic rubber sheet, which comprises treating a. quantity of thedispersion with an aqueous solution of a water-soluble salt of awater-soluble acid in an amount suicient to'increase the size of theparticles in and the stability of the dispersion but insuflicient tocoagulate the dispersion, mixing the so-treated The sheet then istransferred onto an` tity of the aqueous salt solution, intermingling aturbulently-flowing stream of the mixture with a relativelysmallerstream of the untreated dispersion to form further quantities ofstabilized dispersion, admixing the stabilized dispersion with anaqueous acid coagulant to form an aqueous slurry of small syntheticrubber crumbs and v continuously forming the crumbs into a sheet.

3. The method of continuously transforming a dispersion of a syntheticrubber comprising a polymer of a butadiene-1,3, into a synthetic rubbersheet which comprises adding an aqueous acid coagulant to a quantity ofthe dispersion in an amountjust suicient to coagulate the dispersion andform a slurry of discrete synthetic rubber crumbs, mixing the slurrywith a relatively small amount of uncoagulated dispersion, interminglinga turbulently-fiowing stream of the mixture with a relatively smallerstream of the lution of an acid to a quantity of the slurry to` convertthe soap in the slurry to insoluble soapforming acid, in the presence ofan acid-stable protective agent which prevents agglomeration of thecrumbs, to form a slurry containing synthetic rubber crumbs and waterinsoluble soap-forming acid, mixing a relatively small amount of afurther quantity of the soap-containing slurry with the slurrycontaining the water-insoluble soapforming acid, intermingling a.turbulently-flowing stream of the mixture with a relatively smallerstream of a. further quantity ofthe aqueous solution of an acid to formadditional quantities of slurry containing insoluble soap-forming acid,and continuously forming the synthetic rubber crumbs and insoluble acidin the last-mentioned slurry into a sheet.

5. The method of continuously transforming an alkaline soap-containingaqueous dispersion of a synthetic rubber comprising a polymer of abutadiene-1,3, into asynthetic rubber sheet which comprises continuouslyadding an aqueousy acid coagulant to the dispersion to coagulate ythedispersion and form an aqueous soap-containing slurry of small syntheticrubber crumbs, adding additiona1 sufficient aqueous solution of an acidto the slurry to convert the soap in the slurry to water-insolublesoap-forming acid, in the presence of an acid-stable protective agentwhich prevents agglomeration of, the crumbs, and continuously formingthe synthetic rubber crumbs into a sheet.

6. The method of continuously transforming an alkaline soap-.containingaqueous dispersion of a synthetic rubber comprising a polymer of a 10butadiene-1,3 in the form of particles under 0.3 micron in diameter,into a synthetic rubber sheet, which comprises turbulently admixing thedispersion with an aqueous solution of a water-soluble salt of avwater-soluble acid to increase the size of the particles in and thestability of the dispersion, tunbulently admixing the so-treateddispersion with an aqueous solution of an acid' to coagulate thedispersion and form a slurry of discrete synthetic rubber crumbs, andcontinuously forming the crumbs into a sheet.

7. The method of continuously transforming an alkaline soap-containingaqueous dispersion of a synthetic rubber comprising a polymer of abutadiene-1,3, into a synthetic rubber sheet, which comprisesturbulently admixing the dispersion with an aqueous solution of awater-soluble salt of a water-soluble acid and with progressivelyincreasing quantities of an aqueous solution of an acid whereby to forman aqueous slurry containing discrete synthetic rubber crumbs, andcontinuously forming the crumbs into a sheet.

8. The' method of continuously transforming an alkaline soap-containingaqueous dispersion of a synthetic rubber comprising a polymer of abutadiene-1,3, into a synthetic rubber sheet, which comprisesturbulently admixing the dispersion with a suflcient amount of aWater-soluble salt of a water-soluble acid to increase the size of the'particles in and the stability of the dispersion without coagulating thedispersion, turbulently admixing the so-treated ,dispersion with anamount of an acid solution just suicient to co# agulate the dispersionand form a soap-containing slurry of small synthetic rubber crumbs,turbulently admixing the soap-containing slurry with an additionalamount of an acid solution sufficient to convert the soap in the slurryto water insoluble soap-forming acid, and continuously forming thesynthetic rubber crumbs and water insoluble soap-forming acid in theslurry into a sheet.

WHLARD F. BIXBY.

REFERENCES CITED The following references are of record in the ille ofthis patent: e

'(.TN'I'IED STATES PATENTS Number Name Date 1,866,820 Park July 12,1932y 2,040,549 Gammeter May 12, 1936 2,147,293 Hausen Feb. 14, 19392,187,146 Calcott Jan. 16, 1940 2,298,713 'Merrill Oct. 13, 19421,973,059 Gerke Sept. l1, 1934 2,114,758 Young Apr. 19, 1938 2,100,029Gammeter Nov. 23, 1937 2,304,858 Stewart et a1 Dec. 15. 1942 1,879,543Schwerdtel Sept. 27, 1932 2,161,949 Calcott et al. June 13, 19392.305.025 Becker et al Dec. 10, 1942 FOREIGN PATENTS Number Country Date542,204 Great Britain Dec. 31, 1941 Y

